The present invention relates to a frequency-signal transmission system for transmitting an oscillation frequency-signal through power supply lines such as the power supply line and an earth line, such as a frequency-signal transmission system used for an oil level sensor of an automotive vehicle.
In conventional transmission system for such an oscillation frequency-signal, an oscillation frequency-signal from an oscillator section is directly converted into a current and transmitted by a power supply line and an earth line.
When the oscillation frequency-signal is converted into a current signal of sufficiently large current variations and superimposed on power current flowing through the power supply line, the voltage value to be supplied to the oscillator section is undesirably reduced. If the current variations of the converted frequency-signal are reduced in order not to reduce the voltage value supplied to the oscillator section, on the other hand, an external noise or the like causes an operating error. Another disadvantage of the conventional system is that higher harmonics are generated by switching of the oscillation frequency-signal, thereby producing noises in a radio set or computer in the systems mounted on the vehicle.